1. Field of the Invention
The present invention relates to an organic electroluminescent (organic EL) element having an organic material as the luminescent material, and to a process for its manufacture. The invention is particularly suitable for vehicle displays and the like which are exposed to high-temperature environments.
2. Description of the Related Art
Organic EL elements exhibit excellent visibility due to their self-luminescent nature, and allow weight reduction of driving circuits as well because of their low driving voltage of from a few volts to a few dozen volts. They therefore show promise for applications as thin-film displays, lightings and backlights. Organic EL elements are also characterized by abundant color variations.
The basic structure of an organic EL element has a laminate of a plurality of organic thin-films formed on an electrode formed on a substrate, with an electrode formed over the organic thin-film laminate. The major types of materials used for the organic thin-films are low molecular types applied by vacuum vapor deposition and high molecular types applied by coating of solutions onto substrates.
The major types of low molecular materials used are non-crystalline amorphous materials for formation of films by vacuum vapor deposition. These materials exhibit no diffraction peak with analysis by X-ray diffraction.
However, amorphous organic thin-films undergo crystallization when their glass transition temperature (hereinafter referred to as “Tg point”) is exceeded due to temperature variation, resulting in inconveniences such as unevenness of the film producing shorter distances between electrodes, causing current shorts or leaks, and creating electric field condensing.
Techniques aimed at achieving a longer working life for such low molecular organic thin-film materials by means of a crystalline structure are disclosed in Japanese Unexamined Patent Publication HEI No. 3-173095 and Japanese Unexamined Patent Publication HEI No. 5-182764.
The former publication describes the feature of a positive hole transport layer and a luminescent layer in an organic compound thin-film with a crystalline structure, and the examples therein include using N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (hereinafter referred to as TPD) as the positive hole transport layer and an aluminum complex of 8-hydroxyquinoline (hereinafter referred to as Alq3) as the luminescent layer, and employing a substrate temperature of 50° C. during film formation to form a crystalline organic thin-film.
The latter publication describes using the same materials mentioned above, TPD and Alq3, for the positive hole transport layer and luminescent layer, first forming the positive hole transport layer and then the luminescent layer, and immediately following this by heat treatment or performing heat treatment after formation of each layer, to create a fine crystalline aggregate structure of Alq3 as the luminescent layer.
However, when the present inventors used X-ray diffraction to analyze the crystalline states of simple films of TPD and Alq3 formed on ITO (transparent electrode)-formed glass substrates under the conditions described in each of the prior art documents mentioned above, no appearance of crystallinity-indicating diffraction peaks was found. That is, although the aforementioned prior art documents teach that the organic thin-films are crystalline, their crystallinity is such one which does not exhibit diffraction peaks in X-ray diffraction.
The present inventors explain these results as follows. This is illustrated in FIGS. 1A and 1B. FIG. 1A is a structure showing an amorphous state, and FIG. 1B is the presumed crystalline structure according to the aforementioned prior art documents.
Specifically, the lack of diffraction peaks in X-ray diffraction suggests that the crystalline structure described in the aforementioned prior art documents is not that of common crystals with a structure wherein the molecules are arranged in a regular parallel fashion on the substrate, but rather it is a structure with fine crystal aggregates or with fine crystals scattered throughout an amorphous thin-film.
Further examination by the present inventors revealed that, even though the organic compound film structures in these prior art documents are preformed as thin films with a fine crystalline aggregate structure to inhibit heat-induced alteration, formation of the fine crystalline aggregate structure reduces the luminescent efficiency by creating a smaller contact area and lower charge mobility between each of the layers, thereby resulting in reduced luminance or luminance irregularities, and producing new problems of electrode shorts and leaks due to the increased surface irregularities.
The luminance reduction is temperature-dependent, becoming more rapid with higher utilization temperature, while the luminance irregularities occur as light areas and dark areas are produced due to uneven luminance in the element.
According to a first aspect of the present invention, which has been accomplished in light of the new problems discovered by the present inventors, it is an object to realize an organic EL element comprising an organic material which is not amorphous and hence prone to changes in the film, but which has a crystallinity that includes fine crystals, the element being resistant to current shorts or leaks and exhibiting satisfactory luminance characteristics within the range of utilization temperature.
Of the materials used for organic thin-films of organic EL elements, the materials primarily used for low molecular materials, for film formation by vacuum vapor deposition, may be largely classified as either evaporating materials which gasify when in a liquid state, and sublimating materials which gasify when in a solid state.
Generally speaking, most tertiary amine compounds used for positive hole transport layers exhibit volatility, while 8-hydroxyquinoline aluminum complex used in luminescent layers and electron transport and injection layers are sublimating materials. That is, most organic EL elements have a structure comprising a combination of a evaporating material and a sublimating material.
Such organic EL elements exhibit notably reduced luminance at high temperatures and therefore are insufficiently durable as vehicle displays, which are used in higher temperature environments than most other commercial products.
Material manufacturers are therefore developing heat resistant materials, i.e. materials with high glass transition temperatures, and device manufacturers commonly employ methods of film formation using materials with higher glass transition temperatures than the temperatures to which the elements will be exposed. For example, Japanese Unexamined Patent Publication HEI No. 11-3782 teaches that when the environmental temperature exceeds the Tg of the constituent material, crystallization of the constituent material progresses leading to current leaks and shorts.
Experimental research by the present inventors, however, has confirmed that the problem of leaks or shorts is not solved even if the organic thin-film of the element is constructed with a material having a higher Tg than the temperatures to which the organic EL element will be exposed.
In light of these problems, therefore, it is an object according to a second aspect of the invention to provide an organic EL element having an organic thin-film including a luminescent layer between a pair of electrodes, which adequately prevents current leaks or shorts.